1. Field of the Invention
The present invention relates to ripple converters. More specifically, the present invention relates to a ripple converter that maintains stable oscillation for switching regardless of the type or capacitance of an externally attached capacitor for smoothing output.
2. Description of the Related Art
A ripple converter generally refers to a DC-DC converter including a switching element for switching an input DC voltage, a choke coil and a smoothing capacitor for smoothing the switched DC voltage, a flywheel diode for causing a current to flow through the choke coil when the switching element is turned off, and a control circuit for controlling the ON/OFF of the switching element according to the magnitude of ripple in an output voltage. Circuits of such a ripple converter are publicly known, as described, for example, in the document, “Transistor Gijutsu Special No. 28, ” Tokushuu, Saishin Dengenkairo Sekkeigijutsu no Subete, CQ Publishing Co., Ltd., issued on Jul. 1, 1991.
FIG. 14A shows a circuit diagram of a ripple converter that is similar to the circuits disclosed in the document mentioned above. FIG. 14B shows the voltage at an output terminal of a switching element and the waveform of an output voltage (voltage waveform at an output terminal Vout).
A ripple converter 1 according to the circuit diagram shown in FIG. 14A includes a PNP transistor Q1 that functions as a switching element, a flywheel diode D1, a choke coil L1, a smoothing capacitor C1, and a comparing unit 2. The emitter of the transistor Q1 is connected to an input terminal Vin, and the collector thereof is connected to an output terminal Vout via the choke coil L1. The collector of the transistor Q1 is connected to the ground via the flywheel diode D1. The output terminal Vout is connected to the ground via the smoothing capacitor C1. The comparing unit 2 includes a comparator 3 and a reference voltage source Vref having one end connected to the ground. The non-inverting input terminal of the comparator 3 is connected to the output terminal Vout, and the inverting input terminal thereof is connected to the reference voltage source Vref. The output of the comparator 3 is connected to the base of the transistor Q1. Of these elements, the comparing unit 2 functions as a control circuit for performing feedback control of the ON/OFF of the switching element according to ripple in an output voltage.
In the ripple converter 1 constructed as described above, the voltage (output voltage) vo at the output terminal Vout increases when the output of the comparator 3 is at low level (hereinafter abbreviated as L) and the transistor Q1 is ON. When the output voltage vo exceeds a voltage (reference voltage) vref of the reference voltage source Vref, the output of the comparator 3 changes to a high level (hereinafter abbreviated as H) and the transistor Q1 is turned off. Between when the output voltage vo exceeds the reference voltage vref and when the transistor Q1 is turned off, a delay time t1 such as a delay caused by the comparator 3 and a delay due to a switching time of the transistor Q1 exists. Thus, the output voltage vo keeps increasing during the delay time t1.
The output voltage vo starts decreasing when the transistor Q1 is turned off. When the output voltage vo drops below the reference voltage vref, the output of the comparator 3 changes to L and the transistor Q1 is turned on. Between when the output voltage vo drops below the reference voltage vref and when the transistor Q1 is turned on, a delay time such as a delay caused by the comparator 3 and a delay due to a switching time of the transistor Q1 exists. Thus, the output voltage vo keeps decreasing during the delay time t2. When the delay time t2 elapses and the transistor Q1 is turned on the status returns to initial status. This is continuously repeated. It is described that, as a result, the output voltage vo substantially forms a triangular wave and the average value thereof is maintained substantially at the reference voltage vref.
The document mentioned earlier describes that the voltage at the output terminal Vout forms a substantially triangular wave as described above. However, the description is not necessarily accurate. Presumably, it is assumed that a capacitor having a large equivalent series resistance (ESR), such as an aluminum electrolytic capacitor, is used as the smoothing capacitor C1. Actually, in some cases, the voltage forms a substantially triangular wave as described above depending on the characteristics of the smoothing capacitor C1. However, in other cases, the voltage forms a waveform having sudden voltage changes in the vicinities of the peaks of a triangular wave (the timing of the ON/OFF switching of the switching element), or a waveform obtained by alternately folding back quadratic curves, as will be described later.
More specifically, for example, when a capacitor having a relatively large equivalent series inductance (ESL), such as a leaded low-impedance electrolytic capacitor, is used as a smoothing capacitor, the waveform has sudden voltage changes at the timing of the ON/OFF switching of the switching element, as shown in FIG. 15.
As another example, a case where the smoothing capacitor is an ideal capacitor and has sufficiently small ESR or ESL will be considered. In this case, the current that flows through the choke coil increases linearly when the switching element is ON, and decreases linearly when the switching element is OFF. That is, the waveform of the current that flows through the choke coil is triangular. When the smoothing capacitor is an ideal capacitor, the voltage across the smoothing capacitor is a value obtained by integrating the capacitor current. Thus, the voltage across the smoothing capacitor (i.e., the voltage waveform at the output terminal) obtained by smoothing the choke coil current having a triangular waveform has a waveform in which two quadratic curves are alternately connected with each other, as shown in FIG. 16. A peak point in this case is located in the vicinity of the middle of an ON period or OFF period of the switching element.
As described above, the waveform of the output voltage of the ripple converter changes depending on response delays (delay times t1 and t2) of the system and characteristics of the smoothing capacitor.
Generally, when a DC-DC converter made as a module is used, an additional capacitor for output (another smoothing capacitor) is often externally attached to an output terminal of the module. At the stage of designing the module, it is difficult to predict the characteristics of the capacitor attached to the module. Therefore, in some cases, the driving frequency cannot be set to a desired value when the ripple converter is used. This problem becomes particularly severe when a ceramic capacitor having a small ESR or ESL is used as the output capacitor. More specifically, as will be understood from a comparison between FIG. 16 and FIG. 14(b), when a ceramic capacitor is used, the time when the output voltage vo crosses the reference voltage vref is delayed, which causes a decrease in the driving frequency. Thus, a large value must be chosen for the inductance of the choke coil L1, and ripple increases. Therefore, it has been difficult to use a ceramic capacitor as an output capacitor.
As for the response delays of the system, the main cause is a delay caused by the comparator. The delay caused by the comparator is affected by the amount of overdriving (i.e., the voltage difference between the input terminals, or the voltage difference between a maximum value v1 of the output voltage vo and the reference voltage vref in FIG. 14B), and the delay tends to be greater as the amount of overdriving decreases. That is, the delay decreases as the output voltage changes more rapidly and the delay increases as the output voltage changes more slowly.
For example, in the ripple converter 1 shown in FIG. 14A, when the capacitance of the output capacitor is increased in order to reduce ripple in the output voltage, the amount of temporal change in the output voltage decreases. Thus, the amount of overdriving at the time when the time t1 has elapsed since the output voltage crossed the reference voltage vref is less than v1. Thus, the actual delay time is larger than t1, which caused the driving frequency to decrease as compared to a case where the capacitance of the output capacitor is not increased. When the driving frequency decreases, ripple in the output voltage increases. Thus, even when the capacitance of the output capacitor is increased, ripple in the output voltage does not decrease substantially. That is, the driving frequency decreases without reducing ripple.